Journal of Neuro-Ophthalmology, June 1990, Volume 10, Issue 2

The prism dissociation test in detecting unilateral functional visual loss.

The prism dissociation test is useful in quantifying visual acuity in an eye with functional unilateral visual loss, whether severe or mild in degree. A small vertical prism is used to induce diplopia of a 20/20 or larger size projected Snellen letter, and the patient is asked to comment on the quality of each image seen. The acknowledgement of diplopia and comments on the clarity of the dissociated images serve as testimony to the actual level of visual acuity in each eye. In one case, a patient feigned 20/50 visual acuity in one eye after trauma. The prism dissociation test confirmed visual acuity of 20/20 in that eye, as well as excluding an abnormality of contrast sensitivity. In another case, an amblyopic eye previously noted to have 20/30 visual acuity, which allegedly had acute severe visual loss, was documented to have 20/30 visual acuity by the prism dissociation test.

Journal of Clinical Neuro- ophthalmology 10( 2): 127- 130, 1990. The Prism Dissociation Test in Detecting Unilateral Functional Visual Loss Michael L. Slavin, M. D. © 1990 Raven Press, Ltd., New York The prism dissociation test is useful in quantifying visual acuity in an eye with functional unilateral visual loss, whether severe or mild in degree. A small vertical prism is used to induce diplopia of a 20/ 20 or larger size pro­jected Snellen letter, and the patient is asked to com­ment on the quality of each image seen. The acknowl­edgment of diplopia and comments on the clarity of the dissociated images serve as testimony to the actual level of visual acuity in each eye. In one case, a patient feigned 20/ 50 visual acuity in one eye after trauma. The prism dissociation test confirmed visual acuity of 20/ 20 in that eye, as well as excluding an abnormality of con­trast sensitivity. In another case, an amblyopic eye pre­viously noted to have 20/ 30 visual acuity, which alleg­edly had acute severe visual loss, was documented to have 20/ 30 visual acuity by the prism dissociation test. Key Words: Prism dissociation test- Visual acuity­Functional unilateral visual loss. From the Division of Neuro- ophthalmology, Department of Ophthalmology, Long Island Jewish Medical Center, The . Long Island Campus for the Albert Einstein College of Medlcme, New Hyde Park, New York, U. S. A. Address correspondence and reprint requests to Dr. M.. L. Slavin at Department of Ophthalmology, Long Island Jewish Medical Center, New Hyde Park, NY 11042, U. S. A. 127 Functional visual loss is a common clinical entity that may be seen in the setting of frank malinger­ing for secondary gain ( 1), or may constitute a con­version reaction or hysteria due to a deep- seated emotional disorder. At times, abnormal visual acu­ity or visual fields in children may be detected due to lack of concentration or intimidation during test­ing. An early and accurate diagnosis of functional visual loss is critical and may eliminate a needless noninvasive and sometimes invasive work- up. Myriad well- known clinical tests ( 2- 7) are available to the physician in diagnosing functional visual loss. Severe unilateral functional visual loss may often be suspected when a relative afferent pupil defect is lacking in the eye with alleged visual loss. Tests in the naive patient that exploit the fact that visual acuity is normal in the fellow eye ( e. g., hav­ing the patient read through polarized lenses) are often diagnostic. Patients feigning mild unilateral visual loss ( 20/ 30- 20/ 50), on the other hand, may pose a difficult diagnostic challenge. I have found that dissociating the eyes with the use of a small vertical prism ( the prism dissociation test) is a helpful adjunct in uncovering functional visual loss in the latter subset of patients, as well as in those with severe unilateral functional visual loss. THE PRISM DISSOCIAnON TEST A patient with suspected subtle or severe unilat­eral functional visual loss is first asked: " I know that you are having problems seeing with one eye but have you noted double vision?" If the answer is " no" ( and invariably that is the case), I state that vertical diplopia will be induced by the use of spe­cial " optical" devices ( prisms). A four- prism di­opter loose prism is placed base down in front of the normal eye while a 112- prism diopter prism is simultaneously placed in any direction over the eye with the alleged visual loss. In this way, the 128 M. L. SLAVIN patient does not become suspicious that the exam­iner is paying specific attention to one eye or the other. A 20/ 20 ( or larger) size Snellen letter is pro­jected in the distance, while questioning whether vertical diplopia is noted ( Fig. 1). If it is not, quick removal and replacement of the four- prism diopter prism invariably results in an affirmative response. In those stubborn patients who will not admit to diplopia after the above maneuvers, I first intro­duce the four- prism diopter base down in front of the normal seeing eye ( monocularly) from below until the pupil is bisected. This should induce monocular diplopia. Reverting back to binocular testing as above will usually elicit diplopia. In pa­tients with amblyopia due to strabismus, it may be difficult or impossible to induce binocular diplopia ( due to suppression). When the patient admits to diplopia, I then ask whether the two letters seen can be clearly identi­fied and whether they are of equal quality or sharpness. The forthcoming responses to these questions will allow for an accurate appraisal of visual acuity ( as well as relative contrast sensitiv­ity) in the eye in question. Conversely, when pa­tients with genuine visual loss are tested ( e. g., a patient with a unilateral central scotoma and 20/ 80 visual acuity from optic neuritis), the typical re­sponse would be either: " I do not see two images" or " I see two white boxes projected but I can only identify the letter in one of them" ( Fig. 2). FIG. 1. With a four- prism diopter placed base down in front of the normal eye, vertical diplopia is induced. Subjective responses will then allow quantification of visual acuity in the eye with functional visual loss ( see text). FIG. 2. With genuine visual loss. the patient will often deny seeing double or will state that one square ap­pears blank ( see text). ILLUSTRATIVE CASES Case 1 A 52- year- old female school teacher noted di­minished vision in the left eye shortly after being photographed by a student. She alleged that the flash from the camera " blinded" the eye. The left eye was known to have best corrected visual acuity of 20/ 30 in the past due to anisometropic ambly­opia. The teacher filed for compensation, stating that she was unable to read comfortably. On neuro- ophthalmologic examination, visual acuity was 20/ 15 right and 21200 left. Near vision was Jae­ger 1 right and Jaeger 10 left. She was unable to read any of the American Optical- Hardy Rand Rit­ter color vision plates with the left eye. Ophthal­moscopy was unremarkable in each eye. Tests Employed that Indirectly Suggested Functional Visual Loss ( Indirect Tests) 1. Swinging flashlight test: No left relative affer­ent pupil defect was noted. 2. Kinetic perimetry ( Goldmann): The right vi­sual fields were normal, while results on the left revealed a circular 10° field to the V/ 4e isopter. Confrontation visual fields showed severe con­striction in all quadrants almost approaching fixa­tion. There was no expansion of visual fields when tested at 5- 10 ft ( tunnel fields). PRISM DISSOCIATION TEST 129 Tests Employed that May Directly Quantify Visual Acuity ( Direct Tests) 1. Red/ green spectacle test: With a green filter before the eye with alleged visual acuity loss and a red filter in front of the normal eye, the patient is asked to read lightly written red print on white paper ( best written lightly with red pencil) of the 20/ 30- 20/ 50 size. Because the normal eye cannot see red print through a red filter in this setting, a direct assessment of near visual acuity of the fel­low eye is possible. A patient who fails to read the print with the setup as above, but who can read when the filters are reversed, often has legitimate visual loss. In this case, a definitive answer was not obtained because the patient denied seeing the red print irrespective of which eye viewed through the green filter. 2. Prism dissociation test: While viewing a 20/ 20 letter in the distance with prisms placed as stated above, she responded " 1 see two white squares but I can only identify the letter in the upper square." A similar response was offered when a 20/ 25 letter was shown. With a 20/ 30 letter, however, she was able to clearly discern each of the two vertically separated letters. Thus, definitive proof of 20/ 30 visual acuity in the left eye ( unchanged from that reported on previous examinations) was elicited. Case 2 A 62- year- old male gynecologist allegedly struck his left eye against a bathroom sink faucet one morning and developed a vitreous hemorrhage. A peripheral retinal hole was noted and cryotherapy was performed. The vitreous hemorrhage com­pletely cleared, but the patient continued to insist that he was now having problems with " depth perception," and general " focusing" problems and was incapable of performing surgery and laparos­copy. The best corrected left visual acuity noted on multiple examinations was 20/ 50. He received par­tial disability payments for 3 years and now, being age 65, believed that he was justified in receiving lifetime benefits. A desperate insurance company and vitreoretinal surgeon ( who had been complet­ing medical forms during the 3- year period) re­ferred the gynecologist for neuro- ophthalmologic evaluation. On examination, visual acuity was 20/ 20 right and 20/ 50 + 2 left. Left near vision was Jaeger 2. Color vision and perimetry were normal. The left optic nerve, macula, and retina were normal, and the ocular media was clear. Indirect Tests Employed 1. Titmus stereoacuity: Correct responses were given when shown the fly, all animals, and circles 1- 7. The level of stereopsis may be compatible with the stated degree of unilateral visual loss ( 8). 2. A left relative afferent pupil defect was not detected. It was therefore unlikely that an optic neuropathy would account for the 20/ 50 visual acu­ity. Subtle ocular media disturbance or maculopa­thy were not evident on examination. Direct Tests Employed 1. The red/ green test at near ( see above) in this case was not sufficiently sensitive because the vi­sualloss at near was mild. 2. Prism dissociation test: With a 20/ 20 letter on the screen in the distance and a four- prism diopter base- down prism on the right, he stated that he saw double and the " lower" letter was sharper. I then placed the prism base up on the right and he again indicated that the sharper ( and brighter) let­ter was seen by the left eye! There was thus un­equivocal evidence that not only was the visual acuity in the left eye 20/ 20, but that the left eye was indeed the better- seeing eye! DISCUSSION Indirect clinical tests, such as the swinging flash­light test employed in investigating unilateral functional visual loss, alert the examiner that the stated visual symptoms may not be organically based. Of course, a relative afferent pupil defect may be absent when visual loss is due to a subtle maculopathy or ocular media disturbance. Direct testing for functional visual loss ( having the pa­tient unknowingly read with the alleged poor­seeing eye), however, will allow for actual quanti­fication of the visual acuity or visual fields in the eye with alleged diminished vision. The prism dis­sociation test is an additional direct test that may be added to the armamentarium of the ophthal­mologist in diagnosing unilateral functional visual loss, especially when mild. In other such tests, the patient is asked to read binocularly while wearing polarized lenses or colored filters that only allow vision through the eye with the functional visual loss. A not- so- naive patient, of course may close one eye or the other while reading to determine which eye is actually being tested. The prism dis­sociation test, however, introduces the concept of diplopia ( and monocular diplopia, vide supra) of which few patients realize the significance. Subtle I Clin Neuro- ophthalmol. Vol. 10. No. 2. 1990 130 M. L. SLAVIN functional visual loss may be easily diagnosed with this test as seen in case 2. Several prism tests for functional visual loss have been used over the years, although most of them are described in patients feigning severe uni­lateral visual loss, often blindness. Miller ( 2) de­scribed a diplopia test that is similar in design to the prism dissociation test. After an eight- prism diopter prism is placed base down before the non­seeing eye, the patient is asked to view a distant point light. A report of diplopia is evidence that one is not blind. Another test for severe unilateral functional visual loss is based on binocular fusion. If a vertical prism is suddenly introduced in front of an eye with functional visual loss, the patient may have difficulty in reading due to the onset of diplopia. Lastly, if a four- prism diopter base out ( the four- prism diopter base- out test) ( 9) is placed in front of a normal eye viewing a 20/ 20 letter in the distance, a compensatory fixation movement will be noted by the examiner. If the vision in the eye in question is indeed impaired, no such movement will occur. Placement of the same prism in front of the normal fellow eye, however, will result in a bilateral contralateral version. REFERENCES 1. Keltner JL, May WN, Johnson CA, Post RB. The California syndrome: Functional visual complaints with potential eco­nomic impact. Ophthalmology 1985; 92: 427. 2. Miller BW. A review of practical tests for ocular malingering and hysteria. Surv Ophthalmol 1973; 17: 241. 3. Kramer KK, Lapiana FG, Appleton B. Ocular malingering and hysteria: diagnosis and management. Surv Ophthalmol 1979; 24: 89. 4. Keane JR. Hysterical hemianopia. The missing half field defect. Arch Ophthalmol 1979; 97: 865. 5. Gittinger JW. Functional monocular temporal hemianopsia. Am JOphthalmoI1986; 101: 226. 6. Slavin ML. The use of the red Amsler grid and red- green lenses in detecting spurious paracentral visual field defects. Am JOphthalmol 1987; 103: 338. 7. Thompson HS. Functional visual loss. Am J Ophthalmol 1985; 100: 209. 8. Levy NS, Glick EB. Stereoscopic perception and Snellen visual acuity. Am JOphthalmoI1974; 78: 722. 9. Von Noorden GK. Von Noorden- Maumenee's atlas of strabis­mus, third ed. St. Louis: CV Mosby, 1977.